Power screws used in automotive actuators are approximately 40% to 60% in efficiency and are most often not capable of being back driven. Using a ball screw could increase mechanical efficiency into the range of 85% to 95%. Currently, high volume ball screws are more expensive than using a larger motor. However, if a low cost ball screw were available, certain transverse actuators could be made smaller and with higher performance. To accomplish this, ball screws must be approximately one tenth of the current prices.
In a typical ball nut and screw configuration, the elements provide a traveling finite raceway for guiding an endless train of anti-friction balls between their relatively moveable nut and screw. To enable the use of an endless train of balls, it is necessary to return the balls from one end of the raceway to the other end to provide for ball recirculation, the direction of ball return being dependant on the relative directional movement between the relatively moveable inner and outer members.
Previously known balls provide a return tube secured to the outer periphery of the outer member, where the return tube is bent at its ends to extend through accommodating apertures in the outer member to meet with the raceway. While generally satisfactory, such a structure can impose undue restrictions in its application because of the additional radial space occupied by the return tube and because of the possibility of damage to the return tube such as by denting or crushing, which can render the device inoperative. In addition, the apertures for accommodating the ends of the return tube must be located precisely which can raise the cost factor. Furthermore, the cost factor can be raised by the requirement of securing the return tube accurately in place by a securing device, such as clamp.
Alternatively, it has been known to provide a return passageway through the interior of one of the relatively moveable members instead of having the balls lead outwardly to a return tube. It should be apparent that providing such a passageway with the necessary curvature internally of one of the members is difficult and expensive. In an attempt to decrease the difficulty and expense of manufacturing the internal passageway, it has been proposed that a ball return passageway be disposed between the inner and outer members, where the passageway was provided by a channel in one of the members for unloading the balls to either end of the raceway. Ball screw assemblies are commonly used to translate rotary motion to linear motion or vice a versa. The ball nut body in cooperation with the ball screw direct a plurality of ball bearings through an internal bearing race formed between the ball nut body and ball screw in response to relative motion between the ball screw and the ball nut body. The ball bearings translate rotary motion of the ball screw to linear motion of the ball nut body, or translates rotary motion of the ball nut body into linear motion of the ball screw. Both the elongate ball screw and ball nut body commonly include a continuous helical groove which defines the internal bearing race.
Continuous contact between the ball bearings and that portion of the ball screw forming the bearing race causes significant wear of those parts over time. It is known that the components comprising the bearing race must be made from materials that have strength values capable of handling high stresses. Known relationships must be maintained between the helical grooves that recirculate the balls through the ball nut body. The entire ball nut body is commonly manufactured from steel and one or more grooves are precision machined within the ball nut body to provide the required relationship for recirculation. The best known methods of manufacturing steel nut bodies incorporating an internal helical groove result in relatively heavy ball nut bodies which are expensive to manufacture. These single piece or unitary ball nut bodies are normally produced by a mechanical chip-forming machining operation starting from a block of steel. To alleviate the cost of manufacturing steel nut bodies, it has been proposed to form the ball nut body from a fiber reinforced polymeric material. Expansion and contraction of the ball nut body polymeric material due to extreme temperature variances may affect operation of the internal bearing circuits. In addition, the fiber reinforced polymeric material is incapable of withstanding the high stresses imposed on steel ball nut bodies and lacks the desired resistance to wear when subjected to high load use over extended periods of time.
It has also been proposed to provide a ball nut having a composite structure with an inner sleeve preferably constructed by rolling, or by molding as a complete piece or in sectors or by a profiled resilient winding while an intermediate sleeve is preferably constructed by direct molding on the inner sleeve with reinforced plastics or sintered steel. Alternatively, the intermediate sleeve can be constructed separately by molding. In this case two half-shells can be constructed, which are welded together to integrate the intermediate sleeve defining the crossover passageway after mounting on the inner sleeve. The proposed ball nut of composite material also includes an outer sleeve acting as a closing cover for the crossover passage or channel after a sufficient number of balls have been inserted to completely fill the recirculating path.
Another known ball nut configuration includes a metal blank nut having internal helical grooves which are the same as those of a master nut. The master ball nut is used in order to create a silicone imprint mold which includes a positive profile of a portion of the helical groove of the nut and the ball crossover channels properly aligned therewith. With the imprint mold secured in place, a liquid thermal set resin is employed to fill the radial holes and on to the profile of the crossover channels. The resin is allowed to harden as a permanent plug in the radial holes to form crossover members with internal crossover channels which mirror those of the master mold. After the crossover members have hardened, the silicone imprint mold is readily removed from the nut so that the nut can be assembled with a screw and trains of balls in a conventional manner. The silicone imprint mold can subsequently be used with a second ball nut blank to make molded crossover plugs for a second assembly. The plugs are molded from a thermal set resin injected into radially extending apertures, disposed ninety degrees (90°) to the rotational axis of the nut, that have been bored through the walls of the ball nut. The resinous crossover plugs adhere to the walls of the openings after curing so that no additional fastening means are needed to maintain the plugs in position.